The present invention relates to loudspeakers and, in particular, to a dual-chamber loudspeaker, preferably used as a compact subwoofer in a multimedia computer speaker system.
A typical broadband loudspeaker system usually includes separate loudspeakers for providing the different frequency components of the broadband acoustic signal. These separate loudspeakers are coupled together by a suitable crossover network for applying the appropriate frequency component of the electrical input drive signal to each of the loudspeakers.
Most listeners are not able to localize the source of low frequency sounds below about 150 Hz. Accordingly, it is common practice within a typical broadband loudspeaker system to provide only one loudspeaker that operates exclusively below about 150 Hz. This type of loudspeaker is commonly referred to as a subwoofer, and under ideal conditions, its placement remains unnoticeable to the typical listener. Therefore it can be placed conveniently out of sight without compromising the quality of the sound it generates.
An un-mounted or unbaffled subwoofer driver operated in free-air exhibits large mechanical excursions as it approaches its resonant frequency. This undesirable characteristic potentially leads to massively distorted output or even self-destruction of the driver. Moreover, since there is no isolation of the back pressure wave from the front pressure wave in un-mounted subwoofer drivers, the back pressure wave will cancel out the front and produce no bass frequencies. Accordingly, it is customary to mount the subwoofer driver into a housing, so that air in the housing will control this motion.
The use of broadband loudspeaker systems with personal computers is gaining popularity. For example, high fidelity sound is desirable with many multimedia computer applications, such as presentations, games, DVD movies, and the like. Moreover, as the applications for using a personal computer expand, the need for high fidelity sound with these applications will also increase.
The typical personal computer rests on a desk, and customers expect computer-related peripherals to be relatively inexpensive. Accordingly, it is desirable to make multimedia computer-related loudspeaker systems as compact and economical as possible, but without compromising sound quality. Because compactness and economy are desirable, small, wide-band drivers (e.g., 3-inch diameter cone speaker drivers) are commonly used.
Known subwoofer designs are typically expensive to manufacture, too large to be effectively used with a multimedia computer, or fail to effectively suppress sound frequencies above about 150 Hz. For example, the typical subwoofer driver secured to a sealed housing requires a large housing to operate effectively. Accordingly, it neither fits effectively near a computer, nor is it particularly economical to manufacture.
More recently, some subwoofer designs have employed a small driver that is secured within an intermediate partition between front and back chambers of a housing (i.e., a dual chamber housing). Passive resonant devices, such as vent ports, vent tubes and sealed drone cones, pneumatically and acoustically couple the back and front chambers with each other, and the front chamber with the outside environment. These types of systems are commonly referred to as dual-chamber loudspeakers, or loudspeakers having series vented band-pass alignment. U.S. Pat. No. 4,875,546 to Krnan (xe2x80x9cKrnanxe2x80x9d) is an example of a known dual-chamber loudspeaker. In particular, Krnan teaches that undesirable higher frequencies are attenuated without the need for electrical-filtering by appropriately sizing the two chambers, driver, and related interconnecting passive resonant devices therebetween. Krnan notes that the size of each chamber and the mechanical parameters of the passive resonant devices are a function of the xe2x80x9ccut offxe2x80x9d frequency above which acoustic output signals of the loudspeaker are to be attenuated. For optimal results, Krnan teaches that the volume of the back chamber should be related to the volume of the front chamber by a factor of from about 1:1 to 6:1, with optimal performance being achieved with a ratio of about 2.5:1.
Similarly, U.S. Pat. No. 5,025,885 to Froeschle (xe2x80x9cFroeschlexe2x80x9d) teaches that desirable results can be achieved by making the volume of the back chamber xe2x80x9csubstantially smallerxe2x80x9d than the volume of the front chamber.
Dual-chamber loudspeakers, such as those disclosed in Krnan and Froeschle, offer significant improvements over subwoofers having a driver secured within a conventional sealed or vented housing. They are smaller in size, use smaller drivers, are more efficient, and have improved low frequency bass reproduction than a conventional sealed housing subwoofer.
However, while these known dual-chamber loudspeakers advance various theories on how to select the proper size of the chambers and interconnecting ports, they do not teach or suggest the most optimal orientation and construction of the passive resonant devices with respect to each other and the driver. As a result, the size of the chambers, and accordingly, the overall size of the housing, cannot be minimized as small as possible, and sound quality is inadvertently compromised.
In particular, as the overall size of the loudspeaker is reduced, the available volume of the front and back chambers is also minimized. Accordingly, the velocity of air being transmitted though the ports increases, thereby increasing the likelihood of the system generating undesirable high frequency sounds associated with port turbulence, driver excursion limitations, harmonic distortion, and the like. Thus, known dual-chamber loudspeakers must be sized large enough to either minimize these undesirable characteristics, or to include devices, such as a drone cone between the front and back chamber, aimed at reducing the generation and transfer of these undesirable sounds. In practice, the required overall size of the known dual-chamber loudspeakers is often too large to be used effectively in some environments, such as with a multi-media computer loudspeaker system.
Accordingly, the present invention provides an economical and extremely compact dual-chamber loudspeaker, the size of which does not compromise sound quality. It has a relatively small driver received within a partition extending between, and in acoustical and pneumatic communication with, both a front and a back chamber, each of which has a relatively small volume. An elongated vent is in acoustical and pneumatic communication between the front and back chamber at a substantially planar opening in the partition. A sealed drone cone is in acoustical and pneumatic communication between the front chamber and the outside environment at a substantially planar opening in the housing. The two openings are spaced apart and generally parallel to each other, with a portion of the opening in the partition overlapping the opening in the housing, when viewed from the front of the housing.
In a first preferred embodiment, undesirable high frequency sounds associated with driver operation and amplifier clipping are further minimized by directing the driver to face into the back chamber, and the volume of the back chamber is minimized by securing an elongated concentric tube around the port extending between the front and back chambers to achieve the same tuning frequency as a larger chamber. An alternative preferred embodiment includes the centers of the driver and drone cone being aligned, and at least two ports in acoustical and pneumatic communication with the front and back chambers, each port is spaced equal distance from the driver and from each other such that they distribute the pneumatic loads between the front and back chambers evenly, thereby preventing pneumatic forces emanating from the ports from applying asymmetric force to the drone cone.
Additional objects and advantages of the present invention will be apparent from the detailed description of the preferred embodiment thereof, which proceeds with reference to the accompanying drawings.